Ammo size- for an orbital-scale Railgun, rounds ~4-8" in diameter would be appropriate. Especially in reference to the massive sizes of enemy warships.

Ammo type- aluminum would melt from heat ablation. Tungsten is actually the projectile of choice. Depending on the speed of the launch, it may need to be carbon-coated for heat resistance.

Velocity- the speed of sound at Earth's sea level is ~340 m/s. To get an idea of how SLOW that is, most modern firearms fire projectiles at around 600-1100 m/s muzzle velocities. Railguns, on the other hand, fire projectiles much faster. The current fastest projectile launch is a US Navy 90mm Railgun, which fired a slug with the energy of around 32 MJ (I think, I may have to recheck), and a velocity of 2520 m/s. Thats a little more than 1.5 MILES PER SECOND.

However, these defense guns are also intended for LONG TERM ENDURANCE. Hence, A LOT OF AMMO AND POWER STORAGE would be needed. Especially if these things are firing slugs real fast, like ~5000 m/s+.

I think that your wrong, as aluminum hasn't been used for projectiles, well, since ever. Back in the Revolutionary/Civil War eras, it was lead. WWI-II, it was usually lead jacketed in copper. Now, it's usually armor-piercing steel rounds. Aluminum is sometimes used for conventional ammo casings, but NEVER for actually projectiles.

There's also the issue of heat stresses. Aluminum melts REALLY EASY. I mean it. It's also much less dense. For a modern armor-piercing anti-tank weapon system, you need something dense (i.e. HEAVY, more energy to it) and for a Railgun you need something to withstand a TON of heat. As such, Tungsten (or Titanium, though Tungsten is preferable) is the most optimal metal to use. It is the metal with the HIGHEST melting point, it is also very dense, so it is an optimal armor-piercing projectile.

If you use Tungsten but still need additional heat protection, coat it in carbon. But overall, aluminum wouldn't be used.

5000 miles per hour? Let me see, hmm, that's ~27.78 miles per second, which isn't feasible with todays current technology. The USN's current record is a projectile weighing 9 lbs, launched at the velocity of 2520 m/s (~2.5 m/s, which is ~=1.5 miles/second), with an energy of ~32 MJ.

I stand corrected, as I did the math on it just now. Also, 2520 m/s is slightly FASTER than 8270 ft/s. Thus, the weapon is powerful.

i realise ammo is needed... i said in Sins we dont care about ammo... not that Railguns dont use ammo...

the comment about not caring about ammo is because by the same token Vasari Phase Missiles and TEC Missiles and AutoCannons never need reloading... for instance how does a little TEC or Vasari Fighter or Bomber remain useful in combat hours after being deployed from its hangar without needing to dock to reload its weapons...

Its simply a visual thing, but the design could use some more body... its just a little... whispy and unrealistic atm... but the general design is still cool

as for the science of railguns, i think thats pretty much been answered so...

I disagree with aluminum. While it's possible to MAKE it work, it is entirely sub-optimal. Tungsten is much more preferable, because it's denser, which will give it more energy in flight (more penetrative power), and also sturdier. Aluminum is too easily destroyable/meltable. It would likely just hit and crumple.

Tungsten on the other hand would not. With heat, coating Aluminum in CARBON (not Tungsten), would be more cost-effective. Additionally, Tungsten is one of the #1 armor-penetrating rounds used in heavy cannon (90mm+ bore diameter), possibly only superseded by depleted uranium (DU).

ah, I see. Titanium is actually not as good as you'd think. It's good for combining w/ ceramics for armor, but not as good as a AP weapon. DU is by far the most optimal one, but for a magnetic-acceleration weapon Tungsten would be the optimal choice.

I say that because I am unsure as to how conductive DU is.

agreed.

because the definition of a Railgun is that it fires a dense metal slug, and because if it fired plasma then it would be a plasma cannon, and if it fired laser bolts (wtf? what is a laser 'bolt'?) then it would be a laser, not a railgun

hence, why it fires metal slugs.

Fail.

as for whether aluminium would be able to penetrate a shield or hull if moving fast enough, thats true, but a railgun, especially a modern human railgun which isnt all that powerful relatively speaking, would not be able to propel the slug fast enough. micro asteroid damage is caused by small pieces of stellar debris travelling at a significat percentage of the speed of light (or alternatively, being run into by an object travelling at a significant percentage of the speed of light) and THAT is when the damage is caused. if you are travelling at that speed, then a feather or a tooth or a Daddy Long Legs spider could kill you/do damage. However, the slower the projectile or target is travelling, the harder/more dense the slug needs to be to do damage. As such, an aluminium slug travelling at anything less that a percentage of light speed would just crumple. it may give the shields a hard pinch, but they would recover pretty easily.

Evening All,

I have spent some time over the last few days on about 6 different concepts, this is what we have chosen. It's based on parts of midway and the gun outside the Icarus base from the upcoming Stargate Universe. There is now enough space for ammo, power generators and so.

Enjoy

http://riser38.deviantart.com/

I like the new one. It looks much nicer than the 2nd try.

looks awesome now ...